The present invention relates to a catalyst for the selective dimerisation of ethylene or of propylene into monoolefines, based on monovalent Ni.sup.+ nickel on a silica support, as well as its preparation and dimerisation processes conducted in the presence of this catalyst.
In the prior art a certain number of heterogeneous catalysts based on nickel and on silica are already known, which are capable of dimerising olefines. As described by SOHN and OSAKI in the Journal of Catalysis 59, 303, 1979, these catalysts are mostly obtained by co-precipitation in a basic medium of a silica and a nickel salt followed by activation by heating between 100.degree. and 600.degree. C. These catalysts however only show weak activity, since the nickel is inside the silica in a form difficult to reduce.
Applicants have established that a more active catalyst can be obtained by employing a method of preparation enabling precisely the avoidance of passage through a nickel compound which is difficult to reduce.
This catalyst which is the subject of the application for European patent EP No. 008989555 in Applicant's name is characterised by the presence of precursors of sites of catalytic activity, constituted by complexes of monovalent nickel Ni.sup.+ tri-co-ordinated with three atoms of oxygen of the surface of the silica support, this catalyst containing from about 0.01% to about 1% by dry weight of Ni.sup.+, and monovalent nickel Ni.sup.+ complexes can in particular comprise from one to three selective blocking ligands.